1. Field of the Invention
This invention relates generally to a communication system and, more particularly, to a communication system utilizing a time division multiplex (TDM) approach for multiple transmitter broadcasting to mobile users.
2. Discussion of the Related Art
For communication systems in which information is broadcast to mobile users, one challenge in such systems is overcoming signal degradation effects associated with mobile propagation. The mobile user is subject to a time-varying environment that includes attenuation from physical blockages (i.e. buildings, trees, terrain, etc.), as well as multipath and interference effects. One way of overcoming these effects is to increase the transmit power beyond what is required for clear-line-of-sight (CLOS) operation in a multipath-free and interference-free environment. However, due to the large amount of additional transmit power of ten required, this is not always feasible for practical communication systems. This is particularly relevant for satellite-based communication systems broadcasting to mobile users. Although, there is generally less blockage for satellite-based systems than terrestrial-based systems due to the higher elevation angles, it is still difficult in a satellite-based system to transmit enough power to overcome the signal attenuation effects, particularly in outer fringe areas of the coverage region or in urban environments.
Communication systems are also increasingly expected to be capacity efficient and heavy demands are currently required on many different types of digital communication channels. With many of these communication channels, a relatively large amount of bit errors may occur because of the noted physical blockage in a relatively short period of time within a sequence of transmitted bits. Errors occurring in this manner are generally referred to as burst errors, and thus, such communication channels, particularly mobile communication channels, are typically referred to as bursty or fading channels.
Consequently, communication systems operating in the mobile propagation environment have both time and, spacial dependencies. Conventional communication systems may thus employ coding and interleaving, as well as clear-line-of-sight link margins to combat the time and spatially vary mobile propagation environment. Although such techniques may be somewhat effective in this environment, current implementations are not very efficient in terms of the way they utilize the available power, bandwidth and receiver resources.
Traditional coding methods for communication systems operating through a bursty or fading channel often employ some form of interleaving in order to make the communication system more reliable. As is known in the art, interleaving attempts to spread the effect of burst errors in time such that the bit errors are decorrelated and separated from one another. This repositioning of error bits tends to separate the error bits so that they can be processed in conjunction with an encoding and decoding communication system. A convolutional or block decoder is able to tolerate up to some fraction of its input bits degraded or erased, known as the decoder""s erasure threshold, and still provide acceptable performance, measured by bit error probability. The purpose of the conventional interleaver is thus to reduce the probability that the decoder""s erasure threshold is exceeded.
Moreover, for satellite broadcast systems, it is also generally desirable to have complimentary terrestrial transmitters should the satellite transmitters fail. However, the existing communication systems which use both satellite and terrestrial transmitters, such as the European Eureka 147 System or the Global Star Cellular System, each employ a rake receiver which has its disadvantages. Specifically, in each of these systems, the satellite transmitters, as well as the terrestrial transmitters transmit the identical digital data bits using identical RF carrier signals. The rake receiver detects both RF carrier signals and synchronizes both signals to combine each into one robust signal containing the identical digital data. Such systems do provide for a more robust communication system. Although, the simple retransmission of identical digital data bits using identical RF carrier signals is not a flexible approach. In addition, rake receivers are also very complex and costly receivers. Furthermore, time division multiple access (TDMA) communication systems, using a single frequency band have been used where the identical TDMA signals are broadcast from more than one transmitter. However, in each of these systems, which are typically cellular telephone communication systems, the receiver must select a single transmitter to receive from and then hand-off between transmitters when one transmitter signal becomes degraded. Here again, this technique is not a flexible and cost effective way for dealing with burst errors because the receiver may be blocked from receiving signals from a particular transmitter and must then be reconfigured to hand-off to a new transmitter which is not blocked, thereby requiring substantially complex and expensive hardware and software. Moreover, this system also merely transmits the same data bits.
What is needed then is a communication system for broadcasting to mobile users which utilizes a time division multiplex (TDM) approach for multiple transmitter broadcasting which does not suffer from the above-mentioned disadvantages. This will, in turn, provide a more robust communication system; utilize a single frequency band for transmitting redundant or non-redundant information from multiple transmitters to a single receiver in a time division multiplex manner; provide complimentary terrestrial transmitters which operate within the same frequency band as satellite transmitters, broadcast additional local information, relay redundant satellite information to regions which can not receive the satellite signals directly, and which can be received by the, receiver independent of the operation of the satellites; provides a flexible multi-transmitter system with a low complexity and cost efficient receiver; provide a communication system which is dynamic in its allocation of time blocks for each transmitter; increase the quality of signals; provides a reliable communication system to mobile users which minimizes outages; provides a flexible and efficient means of integrating unique and common information into each transmitters broadcast signal; and provides a more robust communication system by using two levels or layers of time division multiplexing. It is, therefore, an object of the present invention to provide a communication system which utilizes a time division multiplex (TDM) approach for multiple transmitter broadcasting to mobile users.
In accordance with the teachings of the present invention, a time division multiplex (TDM) communication system for broadcasting to users is disclosed. The TDM communication system utilizes two (2) levels of time division multiplexing, such that multiple transmitters transmit in a time division multiplexed manner to a receiver which is operable to receive the signals from each of the transmitters.
In one preferred embodiment, a time division multiplex (TDM) communication system for broadcasting to users includes a first transmitter and a second transmitter. The first transmitter is operable to broadcast a first signal into a first coverage area and the second transmitter is operable to broadcast a second signal into a second coverage area, which at least partially overlaps the first coverage region. The first transmitter is allocated a first TDM time block having a first guard time and the second transmitter is allocated a second TDM time block having a second guard time. The first and second TDM time blocks form at least a portion of the TDM repeat interval such that the first transmitter broadcasts the first signal during the first TDM time block and the second transmitter broadcasts the second signal during the second TDM time block.
In another preferred embodiment, a time division multiplex (TDM) communication system for broadcasting to users includes a plurality of transmitters and a receiver. Each of the transmitters is allocated a TDM time block in which to transmit a signal such that the TDM time blocks from the plurality of transmitters forms a TDM repeat interval. The receiver is operable to receive each of the signals from each of the transmitters during each of the TDM time blocks over the TDM repeat interval.
In yet another preferred embodiment, a time division multiplex (TDM) communication system for broadcasting to users includes a channel coding device and a waveform generation device. The channel coding device includes a plurality of first time multiplexers each of which is operable to receive a plurality of channels and operable to time multiplex the plurality of channels into a TDM channel data stream. The waveform generation device includes a plurality of second time multiplexers. The plurality of second time multiplexers is operable to time multiplex each of the TDM channel data streams between a plurality of transmitters. Each of the transmitters is allocated a separate TDM time block forming a portion of a TDM repeat interval.
The use of the present invention provides a communication system which utilizes a time division multiplex approach for multiple transmitter broadcasting to mobile users. As a result, the aforementioned disadvantages associated with the current communication systems have been substantially reduced or eliminated.